Communication systems are known to comprise mobile transmitters and receivers, such as in-car mobile or hand-held portable radios (mobiles), as well as fixed transmitters and fixed receivers, such as base stations or repeaters (fixed end). The mobiles and fixed end are operably coupled by separate transmit and receive communication paths. The communication paths between the mobiles and the fixed end are typically wireless links, such as radio frequency (RF) channels. The communication paths between fixed transmitters and receivers are typically wireline links, such as land-based phone lines.
A typical message within such a communication system may begin with a mobile unit converting an audio signal into a digital data stream suitable for transmission over an RF channel to either another mobile unit or the fixed end. Such systems are often used by public safety institutions, such as local or federal law enforcement agencies. The existence of commercially available radio frequency scanners makes it possible for unauthorized parties to monitor the information transmitted within such a communication system. In efforts to reduce unauthorized eavesdropping, communication systems use digital encryption methods that protect proprietary information transmitted therein.
Digital encryption methods use a known, reversible algorithm to introduce randomness into a digital data stream. To an unauthorized user, an encrypted digital data stream will appear random, and thus unintelligible. Such an algorithm that randomizes digital data is called an encryptor. By necessity, the same algorithm which is capable of encrypting the digital data stream must also be capable of recovering the digital data stream, and hence, is called a decryptor. Often, an encryptor/decryptor algorithm utilizes a dynamic parameter, hereafter referred to as a key, to uniquely specify the nature of the randomness introduced to the digital data stream. Thus, only encryptors and decryptors utilizing an identical algorithm and key are capable of reproducing intelligible messages. Obviously, the security of keys in systems utilizing encryption is of the utmost importance in the prevention of unauthorized monitoring. If the keys of a known encryptor/decryptor algorithm are made available, the ability of unauthorized parties to monitor proprietary communications is greatly enhanced.
Typically, within an encrypted communication unit, the keys used by the encrypted communication unit are stored in a volatile memory device such as RAM (Random Access Memory). This method of storage allows the communication unit to quickly access keys, as might be required for a feature such as encrypted mode channel scanning. Also, the use of a volatile memory allows the key information to be erased in the event that the communication unit loses power or is tampered with, thus maintaining security. For instance, if a communication unit is powered down or tampered with, the information stored in volatile memory is passively erased. Passive erasure typically implies allowing the information stored in volatile memory to decay away with the loss of power. When the communication unit is subsequently powered up, however, an external device is typically required to reload the keys. This requirement can prove to be nuisance in communication units in which power is frequently cycled, such as hand-held portable radios.
A further shortcoming of the use of volatile memory for the storage of key information is that passive erasure of such devices is often unreliable. It has been shown that some RAM devices, despite the sustained absence of power for several minutes (in some cases, hours), may retain some or all of the key information stored therein. This availability of key information could result in a severe breech of security to the entire communication system. Thus, a need exists for a method which allows the use of volatile memory for key storage without the risk of ineffective passive key erasure, and in the event of successful key erasure, without the need for external key reloading.